This INTEGER, marked accessible-for-notify, holds the potential list of stack units that can be targeted by the rlCopySWToUnits trap, and is not meant to be read or set directly like an ordinary object. It matters operationally because it defines which units a software-distribution trap notification can reference when reporting which stack members received a software push. For instance, when software is copied to units 2 and 3 of a stack, this value's bit pattern identifies those specific units inside the resulting trap.